Note: Descriptions are shown in the official language in which they were submitted.
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1 3906
ELECTROSTATIC SPRAY-COATING APPARATUS
Technical Field
The invention relates to an electrostatic spray coating system and more
particularly to an electrostatic spray coating system which provides an
accurate
determination of the high voltage present at a material charging electrode for
use in
regulating the voltage and power available at the electrode.
Back,~round Art
An electrostatic spray gun for spray coating of articles with coating powder
or
coating liquid is known wherein the spray gun includes electrodes for
imparting an
electrostatic charge to the sprayed material and a lamp which lights up
whenever a
specific electric voltage for high voltage electrodes is available. The level
of the high
voltage is not measured or displayed. The spray gun contains a high voltage
generator
composed of a transformer and a voltage multiplier or cascade circuit
according to
"Greinacher" which generates the voltage for the electrodes. The secondary
side of
the transformer is connected to the voltage multiplier circuit while its
primary side
receives a low AC voltage from a suitable source. The low AC voltage is
generated in
practice typically by an oscillator, which may be arranged either in the gun
or
externally to the gun. High voltage generators of this type are well known.
With an increasing current at the high voltage electrode(s), maintaining a
consistently high DC voltage on the high voltage electrodes) requires raising
the input
voltage to the high voltage generator. The output power of the high voltage
generator
corresponds to the mathematical product of the voltage and the current.
Exceeding a
certain level of electric output power gives rise to the risk of electric arc
overs to the
article being coated or to system parts or persons. For that reason it is
known to provide
the high voltage generator with an output limiting circuit. Beginning with a
specific
output power, the output limiting circuit reduces the voltage in response to
current
increases, thereby limiting the output power. Determining the energy requires
determining the current and the voltage at the high voltage electrode(s). In
the past,, the
voltage between the high voltage electrode and ground potential was measured
only in
no load operation, with the voltage turned on but no coating material being
sprayed,
since the coating material would otherwise soil the measuring device and the
spray
pattern on the article being coated would be disturbed by the measuring
device.
Therefore, the customary procedure is measuring the input voltage to the high
voltage
generator and multiplying it by the voltage multiplication factor to arrive at
the
CA 02171253 1999-04-19
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theoretical voltage of the high voltage (HV) electrodes. The prior art
proposes
measurement of the electrode current by measuring the current flowing from the
article
being coated to ground. This current corresponds to the current flowing from
the HV
electrode across the air gap to the article being coated and then from the
article to ground.
But this type of electric current measurement is unwieldy and very inaccurate,
since the
current is very small.
None of the prior art spraying systems allows for the fact that measuring the
input
voltage of the high voltage generator and multiplying it by the voltage
multiplication factor
determines only the theoretical value of the electrode voltage in no load
operation.
1 o However, the calculated value is incorrect during the spraying operation.
In spraying, an
electric current flows in the high voltage generator and causes a voltage drop
due to the
internal resistance in the high voltage generator. The value of the calculated
theoretical
electrode voltage is lower than the actual electrode voltage by the amount of
the voltage
drop in the high voltage generator.
Studies in conjunction with the invention have revealed that with the prior
methods
that measure the input voltage of the high voltage generator and multiply it
by a voltage
multiplication factor, the voltage on the electrodes as calculated thereof is
wrong by up to
30%.
Disclosure of Invention
2 0 The invention sets out to accomplish the objective of determining in a
simple
manner at any time, in no load operation and also during spraying, the actual
value of the
voltage on the HV electrode(s). The actual output voltage is determined more
accurately
than in the past. This allows a more precise control over the maximum output
power at the
HV electrodes which charge the coating material. It also permits more precise
control of
2 5 the voltage on the electrodes) for different coating materials and
different types of coating
or coating thicknesses.
The invention in one broad aspect provides electrostatic spray coating
apparatus
including a spray device for spraying coating material onto an article which
is electrically
connected to ground potential, the spray device having at least one high
voltage electrode
3 o adapted to impart an electrostatic charge to the sprayed material. A high
voltage generator
which includes a voltage transformation circuit is adapted to convert a
relatively low AC
voltage applied to an input circuit to a high DC voltage in an output circuit
which is applied
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to each high voltage electrode, the voltage transformation circuit having a
predetermined
voltage multiplication factor and the output circuit having a predetermined
internal
resistance. The apparatus includes means for measuring the level of the
relatively low AC
voltage applied to the input circuit, means for measuring electric current in
the output
circuit, means for generating an input voltage signal which is a function of
the product of
the measured low AC voltage times the predetermined voltage multiplication
factor and
means for generating a voltage drop signal which is a function of the product
of the
measured current in the output circuit and the predetermined internal
resistance of the
output circuit. Means is responsive to a difference between the input voltage
signal from
1 o the input voltage signal generating means and the voltage drop signal from
the means for
generating a voltage drop signal for determining the actual voltage on each
high voltage
electrode.
Another aspect of the invention pertains to an electrostatic spray coating
process
including spray apparatus in which a low AC voltage is converted in a high
voltage
generator to a high DC voltage in a high voltage circuit which is applied to
an electrode for
charging a sprayed coating material. The method for controlling the spray
apparatus
comprises the steps of determining a voltage transformation factor
representing the voltage
increase from the low AC voltage to the high DC voltage and determining an
internal
resistance for the high voltage circuit, determining the high DC voltage from
the low AC
2 o voltage times the voltage transformation factor less the voltage drop in
the high voltage
circuit resulting from a current in the high voltage circuit and the internal
resistance of the
high voltage circuit and utilizing the determined high DC voltage for
controlling the
electrostatic spray apparatus.
According to the invention, the no load or theoretical electrode voltage is
2 5 determined from the output voltage of the oscillator which is applied to
the voltage
multiplier and the voltage multiplication factor of the transformer and the
voltage
multiplier circuit. This voltage is correct only when there is no electrical
load on the
electrode, i.e. when no coating material is discharged from the spray gun. The
internal
resistance of the high voltage power generator, as seen at the electrode, is
measured while
3 o the high voltage is turned off. During operation of the spray gun, the
electric current to the
electrode is measure and multiplied time the previously measured internal
resistance to
obtain the voltage drop across the high voltage generator. This value is
subtracted from the
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calculated theoretical or no load voltage to obtain an accurate measurement of
the actual
electrode voltage. The measured actual electrode voltage is then used in
regulating the
electrode voltage during spraying and in limiting the power available at the
high voltage
electrode.
Accordingly, the invention seeks to provide an improved electrostatic coating
system.
Other aspects and advantages of the invention will become apparent from the
following detailed description of the invention and the accompanying drawings.
Brief Description of the Drawings
1 o Fig. 1 is an equivalent circuit diagram for a high voltage generator of an
electrostatic spray coating system according to the invention.
Fig. 2 is a schematic diagram of an electrostatic spray coating system
according to
the invention, with a high voltage generator and electronic compensation for
the voltage
drop in the high voltage generator.
Fig. 3 is a schematic diagram of an electrostatic spray coating system with an
electronic control circuit for compensation for the voltage drop in the high
voltage
generator.
Fig. 4 is a schematic diagram of an electrostatic spray coating system with an
electronic control circuit for compensation for the voltage drop in the high
voltage generator
2 o and control of the electrode voltage.
Fig. S is a schematic diagram of an electrostatic spray coating system,
similar to
Fig. 4 and with further details illustrated.
Best Mode For Carrying Out The Invention
An electronic control circuit according to the invention allows, in a simple
2 5 manner, the automatic measurement and control of the relevant actual value
of the
DC high voltage on the high voltage electrode (or high voltage electrodes, in
case of using several) in an electrostatic coating system. The voltage is
accurately
measured and controlled irrespective of whether the distance between high
voltage electrode
and articles being coated changes, irrespective of changes of the electrical
resistance
3 o between the article being coated and ground potential, irrespective of
changes of the
electrical resistance of the coating material on the article being coated as
the film
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thickness or properties of the coating material change, and irrespective of
the quantity
coating material being sprayed.
1. High voltage Generator
According to Fig. 1, a high voltage generator may be viewed as a controllable
voltage supply consisting of an oscillator 2, a transformer 4, and a voltage
multiplier
circuit 6 for generating and applying a required DC high voltage to at least
one HV
electrode 8 of a spray device 10. The spray device 10 may be a hand held spray
gun or
an automatic gun supported by a machine, or a so called rotary atomizer or any
other
form.
Viewing the high voltage generator according to Fig. 1 as a voltage controlled
DC voltage supply, the input voltage is then an alternating voltage, and the
output
voltage is a DC voltage which is generated by the voltage multiplier circuit
6. The
definitions applicable to the equivalent circuit diagram of Fig. 1 for the
high voltage
generator are:
U 1: input voltage or control voltage to the oscillator 2;
U2: high voltage between the electrode 8 and ground potential,
I1: electric current in the oscillator 2;
I2: output current of the high voltage generator (electric current or ground
current from the HV electrode 8 to ground potential);
Av: factor by which the transformer 4 and the voltage multiplier circuit 6
increase (multiply) the AC low voltage U1 of the oscillator 2 to establish the
high DC
voltage U2;
Re: internal resistance of the oscillator 2.
Ri: internal electrical resistance of the spray device 10, measured on the
output end of the voltage multiplier circuit between the HV electrode 8 and
ground
potential;
RL: load resistance (the electric resistance of the air gap between the HV
electrode 8 and an article being coated, the electric resistance of the
article being
coated and of the growing coating applied on it and the electric resistance
from the
article being coated to ground potential);
The equation for the voltage transfer is
Equation 1: U2 = Av * U 1 - Ri * I2
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Equation 1 shows that based on the internal resistance Ri of the voltage
controlled DC voltage supply, depending on the magnitude of I2, the electrode
voltage
drops by a voltage URi.
2. Measuring the DC Voltage on the HYElectrode
5 2. 1 Conventional Method
To measure the actual voltage on the HV electrode 8, the voltage U2 of Fig. l
needs to be measured. During the spraying operation, this measurement can not
be
carried out on the HV electrode 8 from outside the spray device 10. Measuring
the
voltage U2 on the HV electrode 8 from within the spray device 10 requires
electrical
resistors and forming a reference potential, so that the voltage drop can be
measured
via the internal electrical resistors arranged as a voltage divider.
Incorporating such
additional internal resistors for voltage measurement entails the following
disadvantages: additional weight of the spray device 10, additional space
needs for the
internal resistors in the spray device 10, additional cost for the internal
resistors and
with existing spray devices or guns, and the impossibility of retroactively
integrating
internal resistors. Thus, it was not possible for exact measurement and/or
control of
the electrode voltage.
2. 2. Ri Compensation Method
The invention is directed to a method considerably simpler than the
conventional method for determining the electrode voltage. The internal
resistance Ri
of the high voltage circuit in the spray device 10 is compensated for in
determining the
electrode voltage. In Ri compensation, the actual electrode voltage U2 is
calculated
according to equation 1, without requiring a separate voltage measuring
resistor in the
spray device 10. The values Av, U1, Ri and I2 are needed for calculating the
electrode
voltage U2. The factor Av is determined from the winding ratio (primary side
to
secondary side) of the transformer 4 and the number of cascade stages in the
voltage
multiplier circuit 6. The internal resistance Ri of the spray device 10 can be
measured,
and thus determined initially for any spray device, using known measuring
methods.
The measuring proceeds quasi backward, into the spray device 10, by connecting
a
resistance measuring instrument between the HV electrode 8 and ground
potential.
The output current (ground current) I2 and input control voltage U1 can be
continuously measured during operation of the spray device 10 using
conventional
measuring instruments and techniques.
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In place of the equivalent circuit shown in Fig. 1, the real components
illustrating the Ri compensation method are shown in Fig. 2. Fig. 2
schematically
shows an article 12 being coated, a high voltage supply 14, ground potential
16, an
electric reference potential 18 differing from ground potential 16, and an
electric
ammeter 20 for measuring the output current I2 flowing from the HV electrode
8. The
current flows via air gap 22 to the article 12 being coated, then via a
conveyer or
suspension device (not shown) which carries the article being coated, then to
ground
potential 16, and then from ground potential through the ammeter 20 to the
reference
potential 18. The output or ground current, is schematically illustrated in
Figs. 2, 3, 4
and 5 by double dot dash line.
Reference 14 represents symbolically the high voltage supply consisting of the
oscillator, the transformer 4 and the voltage multiplier circuit 6. The
article 12 being
coated, with the coating formed on it, and the air in the air gap 22 contained
between
the HV electrode 8 and the article 12 being coated forms the electric load
resistance
RL. The internal resistance Ri of the spray device 10 is the resistance that
can be
measured, backward into the spray device 10, between the HV electrode 8 and a
second reference potential, normally the ground potential 16. The voltage
supply 14
represents the input or control voltage Ul of the equivalent circuit relative
to Fig. 1.
3. High voltage Measurement with Ri Compensation
With reference to the block diagram of Fig. 3, the following illustrates the
voltage measurement with Ri compensation. The output current (ground current)
I2 is
measured in an electronic control circuit 30 by an output voltage measuring
component 32 and is multiplied electronically by the internal resistance Ri of
the spray
device 10. The output voltage component 32 generates from the multiplication
product a voltage drop signal 34 that corresponds to the internal voltage drop
URi = I2 * Ri across the device internal resistance Ri of the spray device 10.
The input or control voltage U1 is measured by an input voltage measuring
component 36 at the output of the oscillator 2 and is multiplied by the
voltage
multiplication factor Av. The resulting multiplication product U1 * Av
corresponds to
the voltage at the electrode 8 when there is no high voltage current flow I2.
The input
voltage measuring component 36 generates an input voltage signal voltage 38.
An
actual value differential circuit 40 subtracts the voltage drop signal voltage
34 from
the signal voltage 38 and generates a difference signal voltage 42 which in
any
situation of operation, with and without spraying operation under way,
corresponds to
3 5 the relevant HV actual value U2 on the HV electrode 8.
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This type of high voltage measurement with Ri compensation has the following
advantages: there is no measuring resistor needed in the spray device 10;
existing
spray devices 10 in which a measuring resistor cannot be integrated can
nonetheless be
improved according to the invention in such a way that an exact voltage
determination
and voltage control is possible, since the Ri compensation requires no
measuring
resistor in the spray device; and it is low cost and functionally reliable.
4. High voltage Control with Ri Compensation
To maintain a selected electrode voltage at the desired level, the electrode
voltage is determined with the Ri compensation circuit described above and is
passed
on to a controller 44 for controlling the electrode voltage according to Fig.
4. 'The
voltage control is illustrated in the block diagram relative to Fig. 4.
Corresponding to
the electrode voltage U2, the HV actual value 42 is measured as described
above with
reference to Fig. 3 and is subtracted by a set value differential circuit 46
from a high
voltage set value 48 to form a control differential voltage 50. The control
differential
voltage 50 is fed to the input of the controller 44, which generates, as a
function of the
control differential voltage 50, the required control voltage for the
oscillator 44 in
order to establish the desired voltage set value U2 on the HV electrode 8.
5. Output Limitation Display
The voltage generator, consisting of the oscillator 2, the transformer 4 and
the
voltage multiplier circuit 6, must not exceed a specific output power limit
(energy
limit) in accordance with various safety provisions. The power limit is for
reasons of
protection from personal injury and dust or vapor explosion hazard. For
compliance
with these provisions, output power limiting circuits are employed which are
generally
known. Their configuration is such that the electrode voltage is reduced as
the
electrode current increases. The voltage reduction is in keeping with
increasing
electrode current in order to maintain the prescribed energy limit. This means
that the
energy, or output power, of the spray device 10 is being utilized in
accordance with a
voltage current characteristic. A circuit is known in the prior art which,
depending on
an operating point deriving from the electrode current I2 and the load
resistance RL,
detects the arrival at and the entrance in the energy limitation range and
indicates it
optically, acoustically or in another manner. The invention proposes a simple
and
reliable circuit which allows determining for each detection of entering an
output
limitation the maximum possible electrode voltage that will not lead to an
output
limitation. The advantage thereof is that the electrode voltage can be used as
a
3 5 reproducible parameter in assessing the coating result.
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Fig. 4 illustrates schematically the output limiting circuit 52 as a component
of
the electronic control circuit 30. From the block diagram of the voltage
controller
relative to Fig. 4, it follows that the control differential voltage 50 is a
subtraction of
the electrode voltage actual value 42 from the electrode voltage set value 48,
or is a
signal voltage that corresponds to this difference. The output limiting
circuit 52 may
be located in the oscillator 2. As the electrode current I2 rises to a level
where the
output limitation sets in, the output limiting circuit 52 reduces the voltage
U2 on the
HV electrode 8 at continued rise of the electrode current, or output current
(ground
current) I2 in such a way that the output is kept to the permissible output
power, or
energy limit value. Since the electrode actual voltage value 42 is determined,
the
voltage reduction is automatically detected as well. When the output limiting
circuit
becomes effective this way, the voltage on the HV electrode 8 is automatically
pushed
below the HV set value 48. This makes the control difference voltage 50
positive.
The controller 44 now tries in vain to raise the voltage U2, since the output
limiting
circuit 52 prevents a voltage increase. Hence, the control difference signal
voltage 50
remains positive.
According to the invention, the control differential signal voltage 50 is fed
not
only to the controller 44, but also to an electronic comparator circuit 55
that compares
it to a preset reference voltage 54. If the control differential signal
voltage 50 is
greater than the reference voltage 54, the comparator circuit 55 activates a
display 56,
thereby indicating that the HV generator is operating within the range of
output
limitation. The display 56 is preferably a lamp or light emitting diode, but
it might as
well be an acoustic or optical display indicator. The reference voltage 54 may
be the
reference potential 18 or ground potential 16. Thus, the comparator circuit 55
needs to
be activated or cut in automatically only whenever the output power or energy
limit is
reached or exceeded. This will alert an operator of the spray device 10 that
the high
voltage on the electrode 8 is below the set value and, consequently, that the
quality of
the applied coating may be less than desired.
Fig. 5 shows the electronic spray coating system relative to Fig. 4 in greater
detail and employed for spray coating of an article 12 with a coating powder
60. The
coating powder 60 is fed to one end of the spray device 10 by means of a line
62 and is
sprayed from the other end of the device 10 onto the article 12 being coated.
The
article 12 is supported on a carrier 64 which, for example, may be a conveyer
system
which gradually moves a series of articles 12 past the spray device 10. The
conveyer
64 is grounded at 16. The transformer 4 and voltage multiplier circuit 6 are
contained
in the spray device 10. The voltage multiplier circuit 6 preferably is a
cascade circuit
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formed from a plurality of capacitors and rectifiers. A current limiting
resistor 9 may
be contained in the electrical connection 7 between the high voltage output of
the
voltage multiplier circuit 6 and the HV electrode 8. The current limiting
resistor 9
forms a part of the internal resistance Ri of the spray device 10, since it is
measured
automatically in measuring the internal resistance Ri between the HV electrode
8 and
ground potential 16. Instead of one HV electrode 8, several HV electrodes 8
may be
present. Fig. 5, for example, shows two HV electrodes 8, one internal to the
spray
device 10 and one extending external to the spray device 10.
According to Fig. 5, the input voltage measuring component 36 may be an
amplifier circuit. According further to Fig. 5, the output voltage measuring
component 32 may include an electrical measuring resistor 66 connected between
ground potential 16 and the reference potential 18. The output current I2
passes
through the resistor 66 and a voltage drop occurs across the resistor 66 which
is
directly proportional to the current I2. An electric voltage measuring element
68
measures the voltage drop across the measuring resistor 66, and an amplifier
70 that
amplifies the measured voltage. The amplifier 70 generates on its output the
differential signal voltage 34 corresponding to the voltage drop across
internal
resistance Ri of the spray device 10. The oscillator 2 is connected on its
input end to a
low voltage DC voltage supply 72 and generates on its output end 74 a
relatively low
voltage alternating voltage for driving the primary coil 76 of transformer 4.
A
substantially higher AC voltage is created on the secondary coil 78 of
transformer 4.
The voltage multiplier circuit 6 converts the secondary AC voltage to a
considerably
higher DC voltage for the electrode 8. The electrode voltage preferably is
between 4
kV and 140 kV. Identical parts are referenced identically in all of the
drawing figures,
for which reason the remaining components of Fig. 5 matching components of
other
figures are not described separately.
According to the illustrated embodiments, the oscillator 2 for the high
voltage
generator is located in the electronic control circuit 30 and is contained in
a control
cabinet separate from the spray device 10. The oscillator 2 connects via
electric lines
80 to the primary coil 76 of transformer 4 which preferably is located with
the voltage
multiplier circuit 6 in the spray device 10. Along with the transformer 4, the
oscillator
2 forms an oscillator circuit. According to a modified embodiment, however, a
self
vibrating oscillator 2 could be used as well. According to a further modified
embodiment, the transformer 4 and/or the voltage multiplier circuit 6 may be
integrated in the electronic control circuit 30 and may be accommodated in a
control
cabinet separate from the spray device 10. In another embodiment it is
possible to
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accommodate the oscillator 2, in addition to transformer 4 and voltage
multiplier
circuit 6, in the spray device 10. According to a further modified embodiment
it also
would be possible to locate further components, or all components, of the
electronic
control circuit 30 in the spray device, in addition to transformer 4 and
voltage
5 multiplier circuit 6.
The transformer 4 preferably has an adjustable effective voltage ranging
between 0 and 60 volts on its primary side connected to oscillator 2 and has
on its
secondary side an adjustable effective voltage ranging between 0 and 20
kilovolts.
The DC voltage on electrode 8 preferably ranges between 10 kilovolts and 140
10 kilovolts during spraying.
It will be appreciated that various modifications and changes may be made to
the above described preferred embodiment of without departing from the scope
of the
following claims. Although the preferred embodiment has been described for
applying a coating powder, the invention is also applicable to apparatus for
electrostatic application of coating liquids.
